1. Field of the Invention
The present invention relates to an apparatus adapted to estimate a certain kind of information relating to a tired wheel (hereinafter referred to as "vehicle wheel" or "wheel") of a motor vehicle, on the basis of a parameter indicative of the motion of the wheel, and particularly to techniques for improving the accuracy of estimating the wheel information.
2. Discussion of Related Art
For controlling a motor vehicle, it is useful and significant to known the motions and changing characteristics of the wheels of the vehicle, because the wheels are the only components at which the vehicle contacts a road surface. However, it is not easy to directly detect the motions and the changing characteristics of the vehicle wheels during running of the vehicle, since the wheels are attached to the body of the vehicle through a suspension system such that the wheels are displaceable relative to the vehicle body, and since the wheels are rotated.
For a motor vehicle equipped with wheels having tires that are inflated by compressed air so as to maintain a suitable degree of elasticity, for example, the air pressure in the tires has an important effect or influence on the operating characteristics of the wheels. Accordingly, it is desirable to detect the air pressure levels of the wheel tires, but the detection of the tire pressure is also difficult.
The air pressure of a wheel tire may be detected by a pneumatic pressure sensor attached thereto. However, it is not easy to transmit the output of the sensor on the rotating tire to a desired position on the vehicle body while the vehicle is running. It is also possible to determine the tire pressure on the basis of a change in the distance between the road surface and the vehicle body, which change occurs due to deformation of the wheel tire as the tire pressure is lowered. Further, it is possible to estimate a decrease of the tire pressure of one or more wheels as compared with the tire pressure of the other wheels, depending upon the detected rotating speeds of the wheels, based on a fact that the tire pressure decreases with an increase in the rotating speed. However, these methods do not assure accurate detection of the tire pressure. In particular, the latter method does not permit the detection of a decrease of the tire pressure of any wheel if the tire pressure levels of all the wheels whose speeds are detected are lowered.
In the light of the difficulty in detecting a change in the tire pressure of a vehicle wheel, JP-A-62-149502 proposes a tire pressure detecting apparatus which operates depending on a fact that the tire pressure changes with an amount or rate of change of the rotating speed of the corresponding wheel when an external force acting on the wheel varies for some reason, for example, when the wheel runs over a projection of the road surface. Since the maximal value of the amount of change of the rotating speed of the wheel decreases with a decrease in the tire pressure, the tire pressure can be estimated on the basis of the detected maximal value of the wheel speed change amount. The tire pressure detecting apparatus includes signal generating means for generating a signal when the detected amount of change of the wheel speed is higher than a predetermined threshold value, and tire pressure estimating means responsive to that signal, for estimating the tire pressure on the basis of the maximal value of the wheel speed change amount.
However, the condition of the road surface that permits the above-indicated apparatus to detect the tire pressure is limited. Described in detail, the tire pressure can be estimated from the maximal value of the detected wheel speed change amount, only where the relevant wheel runs over a single projection on the road surface. When the wheel passes successive projections and recesses on a bumpy or washboard road surface, changes of the rotating speed of the wheel due to those projections and recesses overlap each other, whereby the maximum value of the detected wheel speed change amount is not commensurate with the tire pressure, making it impossible to effect accurate detection of the tire pressure.
While the difficulty of detecting the tire pressure of the vehicle wheel has been described above by way of example, it has been also difficult to detect other parameters indicative of the motion of the vehicle wheel, such as the angular velocity, angular acceleration, vertical or normal speed, and vertical or normal acceleration of the wheel, and the changing states of the other characteristics of the vehicle wheel and/or tire, such as the eccentricity of the wheel, replacement of the metal wheel member per se, wear of the tire, and attachment of a foreign matter to the tire.
In view of the above situation, the present inventors proposed a wheel information estimating apparatus which includes (a) a detecting device for detecting a wheel motion state or parameter indicative of the motion of a tired wheel of a motor vehicle; (b) a disturbance observer for estimating a disturbance acting on the wheel, on the basis of at least the detected wheel motion state, and a basic value indicative of a specific kind of wheel information about the tired wheel; (c) means for estimating an amount of change from the basic value to a current value indicative of a current state of the wheel information of the tired wheel, on the basis of the disturbance detected by the disturbance observer, and (d) means for estimating the wheel information which is a sum of the estimated amount of change and the basic value indicated above.
The disturbance which acts on the tired wheel is not limited to that given to the wheel from the road surface due to its irregularity. That is, the tired wheel may also suffer from other disturbance components such that those due to a change or variation in the moment of inertia or spring constant of the wheel. The moment of inertia of the tired wheel varies due to wear of the tire, attachment of some foreign matter to the tire, replacement of the tire or metal wheel member with a new one, and the like. The spring constant of the tired wheel varies due to a change of the air pressure of the tire and the like.
As a result of a further study on the wheel information estimating apparatus as described above, the present inventors found the following facts. That is, the disturbance observer is constructed using a dynamic model of a tired wheel. If the dynamic model is required to represent a lot of vibrational modes relating to the wheel, the construction of the disturbance observer becomes complicated, making it difficult to improve its estimating accuracy. On the other hand, the wheel motion state has a frequency characteristic which has a certain relationship with the estimating accuracy of the disturbance observer for estimating the specific kind of information about the tired wheel. The present inventors found that if the frequency characteristic of the wheel motion state is optimally controlled, the estimating accuracy of the disturbance observer can be easily improved, without using such a complicated dynamic model of the wheel.
Another type of wheel information estimating apparatus is proposed in JP-A-5-133831. This apparatus includes a detecting device adapted to detect a rotating speed of a tired wheel as a wheel motion state, and estimating means for estimating an air pressure of a tire as a specific kind of wheel information, on the basis of a frequency characteristic of the wheel motion state, which is received from the detecting device. More specifically, the tire air pressure is estimated on the basis of the frequency of a selected one of a plurality of frequency components of the wheel motion state, which selected one having substantially the greatest strength within a predetermined frequency range. The thus constructed apparatus is referred to as a resonance-frequency type wheel information estimating apparatus.
The present inventors then found that the above-described technique of optimally controlling the frequency characteristic may be effectively applied to the wheel information estimating apparatus of the type as described just above. According to the findings of the inventors, a filter or other device may be interposed between the detecting device and the estimating means, so as to modify the frequency characteristic of the wheel motion state, by particularly emphasizing the selected one of the plural frequency components of the parameter, which is within the predetermined frequency range. The parameter having the thus modified frequency characteristic is applied to the estimating means, with a reduced burden on the estimating means, which leads to improved estimating accuracy.
That is, with respect to the wheel information estimating apparatus which generally includes (a) a detecting device for detecting a wheel motion state, and (b) an estimating device for estimating a specific wheel information on the basis of the detected wheel motion state, the present inventors found it possible to employ the above technique of optimally controlling the frequency characteristic, so as to improve the estimating accuracy of the apparatus, no matter whether the apparatus is of the former type having the disturbance observer, or of the latter type, namely, of the resonance-frequency type.
In the former type of wheel information estimating apparatus having the disturbance observer, the basic value indicative of the specific wheel information is fixed or predetermined, and the disturbance observer estimates the disturbance on the basis of the fixed basic value. A further study by the present inventors on this type of apparatus revealed that the accuracy of estimating the wheel information can be improved by changing or compensating the wheel information basic value so that the amount of change from the basic value to the current value of the wheel information is zeroed, whereby the basic value used by the disturbance observer accurately represents an actual or current state of the wheel information. This method is also effective to improve the estimating accuracy of the wheel information estimating apparatus of the former type.